1. Field of the Invention
The present invention relates to a polycrystalline semiconductor film having excellent crystallinity, and more particularly to a laser heat treatment apparatus for forming a silicon film.
2. Related Art
A pixel portion of a liquid crystal display makes up an image by switching thin film transistors made from an amorphous or polycrystalline silicon film formed on a substrate of glass or composite quartz. However, due to low crystallinity of the silicon film constituting an active layer of the transistors, the thin film transistors are low in performance, typically in mobility. Therefore, it has been difficult to fabricate an integrated circuit requiring high speed and high functionality.
Thus, to realize a thin film transistor with high mobility, a method for improving the crystallinity of a silicon film involves a laser heat treatment of radiating a laser to the silicon film formed on the substrate to enhance the crystallinity.
The pixel transistors in the pixel portion of the liquid crystal panel are driven by a driver circuit that is usually separate, but the driver circuit should be constructed in proximity to the pixel transistors at the same time if high speed and high functionality of the polycrystalline silicon film are to be realized. If this is possible, there is a remarkable advantage in terms of the manufacturing cost and reliability of the liquid crystal panel.
The relationship between the crystallinity of silicon film and the mobility of thin film transistor is described below. The silicon film produced through the laser heat treatment is typically polycrystalline. A grain boundary of polycrystal is made up of lattice defects, which scatter the carriers in the active layer of the thin film transistor to impede the movement. Accordingly, to enhance the mobility of the thin film transistor, it is important to reduce the frequency of carriers traversing the grain boundary in moving through the active layer. Therefore, it is required to decrease the density of lattice defects. The laser heat treatment is aimed at forming the polycrystalline silicon film having large crystal grain diameter and with less lattice defects on the grain boundary from the amorphous silicon film formed on the substrate.
The present inventors proposed an optical system for laser heat treatment in the Unexamined Japanese Patent Application Publication No. Hei 11-179233, in which a laser beam from a laser oscillator is distributed linearly on the surface of silicon film through the optical system, and the linear beam is swept relatively on the silicon film in its orthogonal direction.
FIGS. 4 and 5 are typical views of the laser optical system 3. A laser beam 2 radiated from the laser oscillator 1 is passed through intensity distribution forming means 30 and beam shape forming means 31 to illuminate the silicon film 5 formed on the substrate 50.
The laser beam 2 radiated from the laser oscillator 1 typically shows Gaussian intensity distribution. The intensity distribution forming means 30 preserves the Gaussian intensity distribution in the x direction of the beam, and smoothes the intensity distribution only in the y direction of the beam.
The laser beam with such a top hat distribution is adjusted in the magnification of laser beam length in the x and y directions by the beam shape forming means 31, and the beam shape is made rectangular or linear on the amorphous or polycrystalline silicon film 5. If the longitudinal direction of rectangular laser beam is taken as the y direction, the intensity distribution XC in the x direction on the upper face C of the silicon film 5 has a reduced shape of the intensity distribution XA in the x direction on a plane of incidence A for the intensity distribution shaping means 30, still preserving a property of directivity of the oscillating laser beam 2, while the intensity distribution YC in the y direction is almost uniform on the upper face C of silicon film.
On the other hand, the silicon film 5 as the subject of laser radiation is formed on a silicon oxide film as an under-layer film 51 on the substrate 50 made of glass, which is fixed on a scanning stage, and heated by radiation of the laser beam while the laser beam of rectangular shape is being moved in the x direction.
If the laser beam of rectangular shape is applied onto the surface of the silicon film 5 formed on the substrate while sweeping it in the x direction, the silicon film 5 is heated by absorbing the laser beam, and melted in a rectangular shape. At this time, there is no temperature gradient in the longitudinal direction or the y direction of radiating laser beam because the intensity distribution of laser beam 2 is uniform, but there is some temperature gradient caused by the cooling in the sweep direction or x direction. When the melted silicon film is crystallized by the cooling, the crystal grows in accordance with the temperature gradient, causing one dimensional growth (one directional growth) in the movement direction or the x direction of the substrate 50, so that the crystal grains having a grain diameter of about several μm are formed along the sweeping direction.
If the rectangular or linear beam is employed for the laser heat treatment, the intensity distribution in the width direction of laser beam has a great effect on a recrystallizing growth process, and the intensity distribution in the longitudinal direction governs the area where the crystal grows, whereby an appropriate beam profile and the intensity of radiation must be chosen to fabricate the thin film transistor having excellent characteristics. However, the optical system for forming the rectangular beam has a limited laser output, and to obtain a required intensity of radiation, it is necessary to shorten the length of radiating laser beam in the longitudinal direction, resulting in lower productivity.